You're half right in that second sentence. Both wars should never have been fought.

The US bombed Vietnam, Laos and Cambodia far more than we did Germany in WWII. We had nearly 550,000 troops in Vietnam in 1969. In a country roughly 10% smaller than Montana. Weren't in it to win?

The real similarity in the wars is that at the beginning, the public was all for both wars, for different reasons. In both cases, the US public realized that there was no danger to the US mainland from "the enemy" and we were spending a shitton of money on a war half-way around the world when we had plenty of problems at home to solve.

Obama made it clear in 2016 that the US had no serious military or economic interests in Ukraine, while Russia has both, and that plus Russia being next door, Russia would always have an escalatory advantage. Just like the Chinese supporting the Viet cong.

This is a war that never should have been fought, and which we cannot win without risking WWIII and/or nuclear armageddon.

Why should the US risk that for Ukraine?